The invention concerns a piston pump with a pump housing and a piston, which, with its end surface and the pump cylinder delimits a displacement chamber, which is connected to a hydraulic medium supply via a first check valve and to an operating cylinder via a second check valve, wherein the piston moves axially in reciprocating fashion from one end position of the largest displacement volume to the other end position of the smallest displacement volume against the force of a spring. The invention especially pertains to a hydraulic pump for powering hydraulic tools.
Piston pumps of this type are generally known and also serve to power pressing tools with high and very high pressures. Battery-operated drive motors with limited torque are often used for the mobile operation of such tools. In this context, the maximum pressure that can be produced in a pump of this type is also limited, since the torque produces the maximum force with which the piston can be driven in the pump cylinder.
The task of the invention is to improve a piston pump of the initially mentioned type so that higher pressures can be attained at equal maximum torque. Nonetheless, the pump should be configured in a simple manner.
The problem is solved according to the invention in that the section of the piston that extends into the displacement chamber is provided with an extension with a smaller diameter that connects to the end face of the piston via a shoulder and that has a thickened free end of the piston with a sealing surface facing said shoulder; in that a valve disk, which acts as a seal, is present between the shoulder and the sealing surface, which is brought into the pump cylinder, and through which the connection of the displacement chamber to the second valve is capable of being closed. The axial thickness of the valve disk is smaller than the gap between the sealing surface and the shoulder, so that the valve disk can be moved back and forth between the shoulder and the sealing surface, while the valve disk is provided with at least one opening through which the displacement chamber can be connected with the second check valve and which is closed when the valve disk is brought into contact with the sealing surface. Such a valve disk that is movable on the piston extension can be used to form a pump that operates in two stages, and which creates a high volume flow at relatively low pressures through the movement of the thickened end of the piston in the pump cylinder.
At relatively higher pressures, the return spring cannot completely retract the thickened end of the piston, against the pressure in the operating cylinder, and accordingly also the pressure in the displacement chamber. The displacement chamber further exhibits a stop on which, in a position of the piston between the end positions, the valve disk lies. Through the valve disk a small displacement chamber is formed that is only pressurized with the pressure of the shoulder surface of the piston. The free surface of the shoulder is provided to be smaller than the effective surface of the thickened end of the piston. In this way, the piston can produce higher pressure at the same torque and power. The operating method is described in detail below with reference to the drawings.
It is practical that, if the thickened end of the piston features a circumferential collar, the side of it, which faces toward the step, forms the sealing surface. The spring that rests on the side of the displacement chamber that lies across from the piston can engage the side of the collar that lies across from the sealing surface. In this manner a relatively large and powerful spring can be integrated into the displacement chamber so that correspondingly high pressures can be produced.
According to a preferred embodiment of the invention, the thickened end of the piston is configured as a hat-shaped piston sleeve that is mounted on the extension. Preferably the piston sleeve is mounted on the extension in such a way that it can be removed. In general, the piston unit is made up of three individual parts, the piston with the extension, the valve disk, and the piston sleeve, which are easy and inexpensive to produce. The piston sleeve can also be movable on the extension, as long as it can be ensured that a gap between the valve disk and the sealing surface or the step remains that depends on position. This gap can be very small, as long as the hydraulic medium can flow through it.
The piston can generally be driven as desired. It is practical if the piston is driven by a cam without guide rings. This has the advantage that freewheeling is possible in the direction of the bottom dead center, which is necessary because of the two-stage operating method, since the piston stands still at an intermediate point. The cam only operates during the compression phase in the direction of top dead center.
According to another embodiment of the invention, it is provided that the piston move within a sleeve that is inserted in the pump housing. This has the advantage that the piston unit with the return spring can be installed into a cylindrical displacement chamber of the pump housing, which is restricted by the sleeve in the downward direction. At the same time, this sleeve can form the stop for the valve disk, which is directly retained in the cylindrical recess of the pump housing while acting as a seal.
Generally, it is also possible that the valve disk is guided by the sleeve, set into the pump housing, that extends in the direction of the operating cylinder, while forming the stop for the valve disk. This has the advantage that the sleeve can be optimally fitted to the guide of the piston, on the one hand, and to the guide of the valve disk, on the other. Both measures allow the sleeve to consist of a different material than that of the piston and/or that of the pump housing.